Dissolved Oxygen Dynamics in Coastal Pacific Northwest Rivers: Biological Controls and Management Options

Thursday, 18 December 2014
Daniel J Sobota1, Eugene Foster1, Ryan Michie1 and David Waltz2, (1)Oregon Department of Environmental Quality, Environmental Solutions Division, Portland, OR, United States, (2)Oregon Department of Environmental Quality, Western Region, Eugene, OR, United States
In Oregon’s Central Coast Range (OCR), dissolved O2 concentrations in at least 10% of stream length frequently dip below state standards set to ensure survival and reproduction of native salmonids. We examined O2 dynamics on 12 OCR rivers during times of the year when standards had been violated. Continuous dissolved O2 data were collected 15 minutes apart over a 24-hour period during spring (May - June) or fall (September - November) 2008 on each river. We modeled O2 dynamics for each river with parameters describing O2 exchange with the atmosphere, production of O2 from gross primary production (GPP), and consumption of O2 by ecosystem respiration (ER) fit to observed data. Average nighttime atmospheric O2 exchange and ER were estimated by regressing interval changes in dissolved O2 concentrations between measurements with corresponding O2 saturation deficits. GPP for each daytime sampling interval was calculated as the difference between O2 saturation deficit and the sum of temperature-corrected reaeration and ecosystem respiration. All regression models developed for estimating night-time reaeration and ER were highly significant (p<0.03; adjusted r2=0.17 - 0.77). GPP ranged from 0.62 to 14.95 mg O2 L-1 d-1, ER ranged from -1.21 mg O2 L-1 d-1, and net daily metabolism (NDM; net O2 flux controlled by biological processes) ranged from -11.64 to 3.75 mg O2 L-1 d-1 across all rivers and seasons. Increased aquatic productivity resulting from adjacent and upstream human activities likely altered dissolved O2 dynamics in these rivers. Through scenario analysis, we found that at one river (Alsea), GPP and ER would need to be reduced by 85 and 73%, respectively, to meet the state standard (95% saturation). Our modeling approach can be connected with management actions across a variety of spatial and temporal scales, ranging from local, riparian-scale manipulations of shading and organic matter input to watershed and regional nutrient and temperature management.